The present invention relates to a nonfusion welding process for iron-base ultra-fine grained materials and particularly to welded structural components with distinguished properties, such as strength, corrosion resistance, etc. inherited from the ultra-fine grained materials.
In conventional welding processes for assembling structural components of metallic materials, a welding process for melting parts of a material placed together, edge by edge, is popular.
The structure of the material at the fused parts is broken up by welding to form a solidification structure, and fine grains in the heat-affected zone grow to coarse grains. To suppress such a grain growth to coarse grains in the heat-affected zone as strongly as possible, laser welding (JP-A-11-170088), electron beam welding (JP-A-62-64486), micro-arc welding (JP-A-192838), narrow gap welding (JP-A-2000-246438), etc., all of which can reduce the welding heat input to a base metal, have been proposed.
Furthermore, a welding process, which comprises placing welding pieces together, edge to edge, allowing them to rotate at high speed to effect welding by friction heating (JP-A-2000-015462) has been proposed.
As welding processes without melting of the base metal, on the other hand, solding and brazing processes based on melting of a low melting point filler are now available.
To obtain desirable properties of materials such as strength, corrosion resistance, etc., processes for making crystal grains ultra-fine have been also recently proposed. For example, many iron-base ultra-fine materials having grain sizes of not more than 5 xcexcm are now under development [e.g. The second Symposium on Super Metal (1999): by The Japan Research and Development Center for Metals (a foundational judicial person), etc. and The Fourth Workshop on Ultra-Steel (2000) by National Research Institute for Materials, belonging to Science and Technology Agency, Japan].
When structural components are assembled through joints formed not by soldering and blazing, but by melting of ultra-fine grained materials, the temperature of the welding zone is elevated to the melting point so long as welding is carried out by melting, even if laser welding, electron beam welding, micro-arc welding or narrow gap welding is used as a means of low heat input welding, and consequently the ultra-fine structure with controlled grain sizes will be broken up at the welding zone, and thus growth of such ultra-fine crystal grains to coarse grains is inevitable in the heat-affected zone.
Deformation of welding joints is inevitable in the course of solidification and cooling after the melting. Butt friction welding is not suitable for assembling welded structural components including ordinary plate materials.
An object of the present invention is to provide a nonfusion welding process for iron-base ultra-fine grained materials having the finest grain sizes among the iron-base materials widely used as structural materials, while keeping their distinguished properties of ultra-fine grained materials as inherited to a maximum, and also to provide structural components manufactured by the process.
The present invention provides a process for welding iron-base materials, which comprises welding the same or different kinds of iron-base materials, at least one of which is an iron-base ultra-fine grained material free from any amorphous phase and having an average grain size d in a range of 10 nm less than dxe2x89xa65xc3x97103 nm, by friction stir welding.
The present invention also provides a process for welding iron-base materials, where the iron-base materials are iron-base ultra-fine grained materials free from any amorphous phase, which comprises welding (a) two kinds of the ultra-fine grained materials of chemically and crystallographically same or different kinds, whose average grain sizes d are in a range of 10 nm less than dxe2x89xa65xc3x97103 nm or (b) one kind of the ultra-fine grained material, whose average grain sizes d are in a range of 10 nm less than dxe2x89xa65xc3x97103 nm with another kind or other kinds of the grained materials of chemically and crystallographically same or different kinds, whose average grain sizes d are in a range of 5xc3x97103 nm d, by friction stir welding.
Furthermore, the present invention provides a process for welding iron-base materials, where the iron-base materials are austenitic stainless steel, which comprises welding (a) two kinds of the stainless steel from one another, whose average grain sizes d are in a range of 10 nm less than dxe2x89xa65xc3x97103 nm, or (b) one kind of the fine grained stainless steel, whose average grain sizes d are in a range of 10 nm less than dxe2x89xa65xc3x97103 nm, with another kind of the stainless steel, whose average grain sizes d are in a range of 5xc3x97103 nm less than 3, by friction stir welding.
Still furthermore, the present invention provides a process for welding iron-base materials, where the iron-base materials are ferritic stainless steel with not more than 0.12 wt. % C, free from any of martensite phase and tempered martenite structure, which comprises welding (a) two kinds of the stainless steels, whose average grain sizes d are in a range of 10 nm less than dxe2x89xa65xc3x97103 nm, or (b) one kind of the fine grained steel, whose average grain sizes d are in a range of 10 nm less than dxe2x89xa65xc3x97103 nm, with another kind of the stainless steel, whose average grain sizes d are in a range of 5xc3x97103 nm less than d, by friction stir welding.
In the present invention, other iron-base materials such as two-phase stainless steel, ultra-fine grained austenitic stainless steel, ferritic stainless steel, etc. can be likewise used for the welding, as will be mentioned later.
In the present invention, as the iron-base materials there can be used two-phase stainless steel, fine grained materials of austenitic stainless steel, ferritic stainless steel, two-phase stainless steel, etc. as explained below.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.